Operational amplifiers typically require high open-loop gain and high bandwidth to minimize errors in output voltage. Obtaining high open-loop gain is difficult in modern MOS amplifiers, however, due to the small intrinsic gain of short-channel MOS transistors. Long-channel transistors can improve gain, but bandwidth is sacrificed.
Positive feedback or cascode amplifiers, such as regular cascode, double cascode, and active cascode, can improve gain, but the output swing and input common-mode range are concomitantly reduced, as is generally known in the art. The reduction in output swing (often termed "compliance" or increase of "headroom" relative to the power supply) is a problem, particularly in low voltage applications. In this context, another problem with using cascode circuits for improving gain is that the output resistance of the amplifier is increased, and this makes it more difficult for the amplifier to drive a resistive load.